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Lecanium scale found in blueberry fields

Editor’s note: This article is from the archives of the MSUCrop Advisory Team
Alerts. Check the label of any pesticide referenced to ensure your use is
included.

Lecanium
scale has been found in blueberry fields this summer, especially around
the Holland/West Olive region. They have also been found in some
vineyards in the northwest region. The waxy scales are small and brown
(3-4 mm long) and typically on the new growth from last year. The high
densities of scale will cause honeydew on the foliage and fruit beneath
the colonies, and this can cause problems with growth of sooty mold.
Additionally, there may be reduced growth of new foliage on shoots that
are heavily-infested.

Our
earlier article on this subject highlighted the need to wait until
crawlers emerge from under the protective scale before applying
treatments to protect bushes, and we have now observed crawler movement
from the adult scales. In our weekly sampling for scale through June,
the average number of scale crawlers on sticky tape traps were as
follows: zero on June 2, 0.25 on June 10, 0.2 on June 14, and 49.1 on
June 22. This rapid increase in scale crawler density indicates that
this week would be the right time to protect fields where high scale
density was found this spring. This will prevent scales from settling on
this year’s growth and continuing their development.

Scouting
to identify the start of crawler emergence can be done in your own
fields by weekly checking of double-sided sticky tape placed near scale
colonies, or by regular checking of colonies with a 10x hand lens. The
scale crawlers will be visible around the adult scales, moving across
the vine shoot surface. They are about 0.5 mm long and yellow.

We
have already seen parasitic wasps emerging from scales collected in crop
fields this spring, and fungal pathogens have also attacked colonies in
some wooded areas near blueberry fields, so some natural biological
control is underway. In recent sampling, scale populations in woods had
less than 5 percent survival, but each surviving scale can produce
hundreds of crawlers, so they have a high potential for reproduction.

Once
scale crawler emergence is identified, protection of bushes is possible
using effective insecticides. We have conducted a small demonstration
trial in a West Olive farm this spring, comparing scale control in a
field receiving these three programs for fruitworm control and measuring
the control of scale. These programs included two applications applied
14 days apart after bloom 1) Guthion and then Imidan, 2) Assail applied
twice, 3) Mustang Max applied twice. All of these programs provided
complete control of scale, with no surviving adult scale a week after
the final treatment. These results and the further details below
indicate that many growers managing blueberry maggot and Japanese
beetles at this time should be achieving control of Lecanium scale. For
this pest, it will be important to get coverage through the canopy
including the lower branches to make sure all the new young growth is
protected.

The
landscape entomologist at MSU, Dr. Dave Smitley, who has plenty of
experience with Lecanium scale in shade trees recommends use of
neonicotinoids or pyrethroids for control of Lecanium scale crawlers.
Neonicotinoids include Provado, Assail, Actara and Scorpion and these
products are systemic, being absorbed into the plant tissues after
application and therefore resistant to washoff. Pyrethroids such as
Danitol, Mustang Max, and Baythroid will provide quick knockdown of the
scale crawlers. However, pyrethroids can also disrupt biological control
for this and other pests, so they should be used with care. Dr. Smitley
has also mentioned to us that oils have not worked very well against
soft scales such as Lecanium scale, but that option may be something
organic growers would be interested in trying next spring at
delayed-dormant timing to try and reduce scale populations on bushes.

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